Peak reduction for simulcast broadcast signals

ABSTRACT

A reduced peak simulcast signal generator and a method for generating a stable peak-attenuated simulcast signal, which combines a digital broadcast signal and an analogue broadcast signal, are described. The simulcast signal generator comprises at least one attenuation stage for attenuating the digital and/or the analogue broadcast signal in order to obtain a stable simulcast signal. Each attenuation stage comprises a simulcast signal generator for generating an error signal, and attenuation means which attenuate, in dependence of said first error signal, at least one of the analogue and the digital broadcast signal.

[0001] The present invention is related to a reduced peak simulcastsignal generator, to a method for generating a peak-attenuated simulcastsignal, and to a simulcast broadcast signal.

[0002] DRM (Digital Radio Mondial) is a digital service in thebroadcasting bands below 30 MHz. A simultaneous transmission of theconventional analogue and the new digital service will simplify andaccelerate the introduction of the DRM system, since listeners of theanalogue service are not lost during the introduction of DRM.

[0003] In the European Patent Application EP-A-01118908.1, “DRM/AMSimulcast”, the simulcast transmission signal and simulcast signalgenerators are described in detail. The complete disclosure of saidapplication, which has been filed by the applicant of the presentapplication, is herewith incorporated into this specification byreference.

[0004] When a digital DRM signal and an analogue audio signal aretransmitted simultaneously, via one channel in the simulcast modedescribed in EP-A-01 118 908.1, the system will become instable in casethe AM peak amplitude or the DRM peak amplitude exceeds a certain value.Because of this behaviour, the signal power of the analogue signal andthe DRM signal must be small compared to the carrier. As a result, thesignal power of the AM signal and the DRM signal is relatively small.

[0005] It is therefore an object of the invention to provide a simulcastsignal generator, a method for generating a simulcast signal and asimulcast broadcast signal which permit to increase the signal power ofthe analogue and digital signal in order to increase the coverage areaof the analogue and the digital system.

[0006] The object of the invention is solved by a reduced peak simulcastsignal generator according to claim 1, by a method for generating apeak-attenuated simulcast signal according to claim 9, and by asimulcast broadcast signal according to claim 17. Preferred embodimentsthereof are respectively defined in the respective following subclaims.A computer program product according to the present invention is definedin claim 20.

[0007] According to the invention, a reduced peak simulcast signalgenerator for a digital broadcast signal and an analogue broadcastsignal, comprising an initial attenuation stage, is provided. Thereduced peak simulcast signal generator comprises a final simulcastsignal generator, which generates a final simulcast signal fortransmission. Furthermore, said initial attenuation stage comprises afirst simulcast signal generator, which generates, starting from saidbroadcast signals, a first error signal, and first attenuation means,which attenuate, in dependence of said first error signal, at least oneof said broadcast signals in order to generate first attenuated signals,based on which said final simulcast signal is generated.

[0008] Usually, the peak amplitude is decreased using a limiter. Thislimiter leads to a broad spectrum and is therefore not suited for thegeneration of a simulcast transmission signal.

[0009] The invention allows to generate a stable simulcast signal with amaximum signal strength of the underlying analogue and digitaltransmission signals. The amount of attenuation is chosen in dependenceof the error signal, which is a measure of the signal stability of thesimulcast signal. Just as much attenuation as necessary is applied tothe underlying signals.

[0010] In order to attenuate the analogue and/or the digital broadcastsignal, smooth weighting functions are used, and thus, spectraldistortions can be avoided. The simulcast signal is only attenuated whenit is indispensable in order to perform the sideband modulation of theanalogue and the digital transmission signal. Most of the time, thesignal amplitude is not affected.

[0011] According to a preferred embodiment of the invention, said firstattenuation stage comprises delay elements for delaying said broadcastsignals by respective time delays that compensate the processing timecaused by said first simulcast signal generator and said firstattenuation means. When the processing time spent by the first simulcastsignal generator and the first attenuation means is exactly compensated,further signal attenuations will be exactly in phase with former signalattenuations. Thus, the degree of peak reduction can be exactly tailoredto the needs of the respective transmission situation. Distortionscaused by phase shifts are avoided.

[0012] According to a preferred embodiment of the invention, saidreduced peak simulcast signal generator further comprises M additionalattenuation stages, with each additional attenuation stage comprising: asecond simulcast signal generator, which generates, starting frompreceding attenuated signals, a second error signal, second attenuationmeans, which attenuate, in dependence of said second error signal, atleast one of said preceding attenuated signals in order to generatesecond, further attenuated signals, based on which said final simulcastsignal is generated. Thereby M is a positive integer including zero andis determined according to the desired stability of the final simulcastsignal and/or according to the degree of attenuation of each attenuationstage. By means of the additional attenuation stages, the requiredamount of signal attenuation for obtaining a stable simulcast signal canbe found with a cascade of different available attenuations. If acertain attenuation is not sufficient, the corresponding error signalwill indicate that the simulcast signal is not stable, and theattenuation can be increased by activating additional attenuationstages. An iterative strategy is important for obtaining the optimumsimulcast signal. When a stable simulcast signal is achieved, one can besure to have the highest possible signal strength within said simulcastsignal. The resulting advantage is that the coverage area of theanalogue and the digital system is increased.

[0013] Preferably, said additional attenuation stages comprise delayelements for delaying the preceding attenuated signals by respectivetime delays that compensate the processing time caused by said secondsimulcast signal generator and said second attenuation means. Thus, allthe signals in the simulcast signal generator remain in phase, anddistortions are avoided. Furthermore, different attenuations caused bydifferent attenuation stages can be superimposed in a well-definedmanner.

[0014] Preferably, said peak reduction stages comprise multipliers formultiplying at least one of said broadcast signals or said precedingattenuated signals with a weighting signal. A multiplication withweighting signals enables a straightforward accumulation of differentweighting procedures. Using a smooth weighting signal does not lead tospectral distortions.

[0015] Preferably, said digital broadcast signal is a DRM signal.Further preferably, said analogue broadcast signal is an amplitudemodulated audio broadcast signal. At this moment, the technicaltransition from conventional AM technology to digital standards such asDRM is taking place. For this reason, a modulation technique that iscapable of supporting both standards simultaneously is highlyappreciated.

[0016] Preferably, said first and second simulcast signal generatorrespectively comprise a first modulator to modulate a digital signal toone sideband of a carrier of the transmission channel, a secondmodulator to modulate a correcting signal to the other sideband of thecarrier of the transmission channel, whereby said correcting signal isdetermined such that the envelope demodulation of the transmissionchannel represents the simulcast signal generator's analogue inputsignal, and a first adder for adding said both sidebands.

[0017] The inventive method generates a peak-attenuated simulcast signalstarting from a digital broadcast signal and an analogue broadcastsignal. The method comprises a first step of generating a first errorsignal, by means of a first simulcast signal generator, from saidbroadcast signals. The next step is to determine if said first errorsignal exceeds a predefined threshold, and in case said first errorsignal is too large, to attenuate at least one of said analogue and saiddigital broadcast signals in order to generate first attenuated signals.Last, a final simulcast signal for transmission is generated based onsaid first attenuated signals.

[0018] Preferably, a simulcast broadcast signal combining a digitalsignal and an analogue signal in one transmission channel can begenerated by the method described above.

[0019] The invention does not have to be implemented in hardware. Theinvention can also be realised as a computer program product whichcarries out the method steps as described above when said computerprogram product is executed on a computer, digital signal processor orthe like.

[0020] Further objects and features of the present invention will becomeapparent from the following description of an exemplary embodimentthereof taken in conjunction with the accompanying figures, wherein

[0021]FIG. 1 shows a block diagram of a peak-attenuated simulcast signalgenerator comprising (M+1) attenuation stages;

[0022]FIG. 2 depicts the weighting signal, which is used for attenuatingthe analogue broadcast signal, as a function of time,

[0023]FIG. 3 shows a block diagram of a single simulcast signalgenerator comprising N correction signal units;

[0024]FIG. 4 shows the modulation of the lower sideband and the uppersideband for a simulcast broadcast signal.

[0025] In FIG. 1, a block diagram of the simulcast signal generationunit according to the invention is shown. In order to avoid that largeAM peak amplitudes or large DRM peak amplitudes cause an instablesimulcast broadcast signal, the analogue broadcast signal 1 and thedigital broadcast signal 2 may be attenuated as much as necessary toobtain a stable signal. For this purpose, an iterative procedure issuggested, and therefore, several attenuation stages, an initialattenuation stage 3 and additional attenuation stages 4, are provided.

[0026] The initial attenuation stage 3 comprises a simulcast signalgenerator 5 and a peak attenuation unit 7. Both the analogue broadcastsignal 1 and the digital broadcast signal 2 are connected to thesimulcast signal generator 5. The simulcast signal generator 5transforms the analogue and the digital broadcast signal 1, 2 into asimulcast signal. The digital broadcast signal 2 modulates the uppersideband of the simulcast signal, whereby the lower sideband ismodulated according to a correcting signal. The error that occurs whenthe simulcast signal is generated is indicated by an error signal 6.Especially in case of large signal amplitudes, the simulcast signal isnot stable any more. In this case the error signal 6 assumes a largevalue.

[0027] According to the invention, the simulcast signal is stabilized byattenuating the analogue and/or the digital broadcast signal 1, 2. Theerror signal 6 is forwarded to the peak attenuation unit 7. The degreeof attenuation is chosen according to the magnitude of the error signal6. Alternatively, a constant attenuation degree can be chosenindependent from the magnitude of the error signal. In case the errorsignal 6 has a small magnitude, there will be no attenuation at all. Incase of a large error signal 6, either the analogue broadcast signal 1or the digital broadcast signal 2 (or both) is attenuated. The errorsignal 6 is compared with a predetermined threshold, and in case theerror signal 6 exceeds said threshold, at least one of the analogue andthe digital broadcast signal 1, 2 have to be attenuated. In the solutionshown in FIG. 1, only the analogue broadcast signal 1 is attenuated; thedigital broadcast signal 2 is not attenuated at all. The attenuation ofthe analogue signal usually leads to sufficient results. In case of highpeak amplitude of the DRM signal, the peak amplitude of the DRM signalcan be attenuated in the same way as it has been described for theanalogue signal.

[0028] The peak attenuation unit 7 generates a weighting signal 8, whichis forwarded to the multiplier 11. The delayed analogue signal 9, whichhas been delayed by the delay element 10, is forwarded to the multiplier11 as well. At the multiplier 11, the delayed analogue signal 9 ismultiplied with the weighting signal 8, and as a result, the attenuatedanalogue signal 12 is obtained.

[0029] The weighting signal 8 is depicted in FIG. 2. Usually, theweighting signal has the amplitude 1. At the time t=0.25 sec, the peakattenuation circuit has detected that the error signal exceeds a certainthreshold. The weighting signal reduces the gain from 1 to 0.9 at thetime 0,25 sec. So the peak amplitude of the audio signal that caused thedistortions in the AM demodulated simulcast signal is attenuated, andthe attenuated signal is then used for a new attempt to obtain a stablesimulcast signal.

[0030] Within the simulcast signal generator 5 and the peak attenuationunit 7, some processing time is spent for evaluating the error signal 6and for providing the weighting signal 8. Therefore, both the analoguebroadcast signal 1 and the digital broadcast signal 2 are delayed inorder to compensate this time delay. The delay element 10 converts theanalogue broadcast signal 1 into the delayed analogue signal 9, and thedelay element 13 converts the digital broadcast signal 2 into thedelayed digital signal 14.

[0031] Next, the attenuated analogue signal 12 and the delayed digitalsignal 14 are forwarded to the M additional attenuation stages 4,whereby M is a positive integer including zero. Both the attenuatedanalogue signal 12 and the delayed digital signal 14 are input to thesimulcast signal generator 15 ₁, which is part of the first additionalattenuation stage. Starting from this set of attenuated signal 12 and14, the simulcast signal generator generates a simulcast signal and thecorresponding error signal 16 ₁, whereby the error signal indicateswhether it has been possible to integrate the analogue signal 12 and thedigital signal 14 into one simulcast signal. The error signal 16 ₁ isforwarded to the peak attenuation unit 17 ₁ of the first additionalattenuation stage, and said peak attenuation unit 17 ₁ provides theweighting signal 18 ₁ to the multiplier 19 ₁. The attenuated analoguesignal 12 is connected to the delay element 20 ₁, in order to obtain thedelayed signal 21 ₁. The delayed signal is connected to the multiplier19 ₁, which multiplies said delayed signal 21 ₁ with the weightingfunction 18 ₁. The degree of attenuation is chosen according to themagnitude of the error signal 16 ₁, or a fixed degree of attenuation ischosen. As a result of the multiplication, the further attenuatedanalogue signal 22 ₁ is obtained, which is forwarded to the nextattenuation stage.

[0032] The delayed digital signal 14 is also forwarded to a delayelement 23 ₁, and thus, the delayed digital signal 24 ₁ is obtained. Thefurther attenuated analogue signal 22 ₁ and the delayed digital signal24 ₁ are passed on to the next attenuation stage, and the iterativeattenuation procedure is continued.

[0033] At the last attenuation stage (which is the M^(th) additionalattenuation stage), an attenuated analogue signal 25 and a delayeddigital signal 26 are generated. Those two signals 25, 26 are forwardedto the final simulcast signal generator 27, and said final simulcastgenerator 27 determines the final simulcast transmission signal 28.

[0034] In FIG. 3, a block diagram of an exemplary simulcast signalgenerator is shown. A technical implementation of a simulcast signalgenerator can be found in the above-referenced European PatentApplication EP-A-01118908.1, “DRM/AM Simulcast”. The simulcast signalgenerators 5 and 15 ₁ to 15 _(M) can be implemented as shown in FIG. 3.The simulcast signal is generated by a first modulator 31 that modulatesthe digital transmission signal to one sideband of a carrier of thetransmission channel, here the upper sideband USB, and a secondmodulator 32 that modulates the correcting signal C to the othersideband of the carrier of the transmission channel, here to the lowersideband LSB. The resulting signal is depicted in FIG. 4, in which thetwo different sidebands are shown.

[0035] In order to generate the simulcast signal, the first adder 33adds said both sidebands and possibly the carrier signal at the centreof both sidebands, whereby the carrier signal is generated by a carriersignal generator 34.

[0036] The digital transmission signal supplied to the first modulator 1corresponds to the correcting signal C in time, i.e. is delayed duringthe generation of the correcting signal C to match therewith.

[0037] The simulcast signal generator shown in FIG. 3 further comprisesa correcting signal generator that—in this case—also delays the digitalsignal. The correcting signal generator comprises a third modulator 35to modulate said digital transmission signal to said one sideband, i.e.here to the upper sideband, a fourth modulator 36 to modulate theanalogue transmission signal to the other sideband, here the lowersideband, a second adder 37 to add said both sidebands and the carriersignal which is generated by a second carrier signal generator 38 at thecentre of said both sidebands. Further, the correcting signal generatorcomprises a first envelope demodulator 39 to envelope demodulate the sogenerated simulcast signal, a first delay element 40 to delay theanalogue transmission signal according to the processing time togenerate a demodulated respective simulcast signal, a first subtractor41 to generate an error signal by subtracting the envelope demodulatedsimulcast signal from the delayed analogue transmission signal, a firstmultiplier 42 to weight the error signal with a time-variant ortime-invariant variable, a third adder 43 to generate the correctingsignal C or an intermediate correcting signal by adding the weightederror signal to the delayed analogue transmission signal, and a seconddelay element 44 to delay the digital transmission signal according tothe processing time needed to generate and demodulate the respectivesimulcast signal.

[0038] As stated before, these elements of the correcting signalgenerator which can be regarded as one correcting signal unit eithergenerate the correcting signal or an intermediate correcting signal. Thecorrecting signal generated by this one correcting signal unit isnormally only a rough estimation and would not necessarily lead to aproper transmission to the analogue transmission signal. However, thequality thereof might also be regarded as sufficient. In case thisshould not be sufficient, the resulting signal supplied from the thirdadder 43 might be processed in the same way as the analogue transmissionsignal was processed before by N similar following correcting signalunit(s). In this case also the digital transmission signal will getaccordingly delayed. Such a repetition of the processing might beperformed N times, i.e. by N similar following correcting signalunit(s), wherein N is a positive integer including zero and isdetermined according to the wanted accuracy of the correcting signalwhich is determined after the N^(th) correcting signal unit.

1. Reduced peak simulcast signal generator for a digital broadcastsignal (2) and an analogue broadcast signal (1), with a final simulcastsignal generator (27), which generates a final simulcast signal (28) fortransmission, characterized by an initial attenuation stage (3)comprising a first simulcast signal generator (5), which generates,starting from said broadcast signals (1, 2), a first error signal (6),and first attenuation means (7), which attenuate, in dependence of saidfirst error signal (6), at least one of said broadcast signals (9) inorder to generate first attenuated signals (12, 14), based on which saidfinal simulcast signal (28) is generated.
 2. Reduced peak simulcastsignal generator according to claim 1, characterized in that said firstattenuation stage comprises delay elements for delaying said broadcastsignals by respective time delays that compensate the processing timecaused by said first simulcast signal generator and said firstattenuation means.
 3. Reduced peak simulcast signal generator accordingto claim 1 or 2, characterized in that said reduced peak simulcastsignal generator further comprises M additional attenuation stages, witheach additional attenuation stage comprising: a second simulcast signalgenerator, which generates, starting from preceding attenuated signals,a second error signal, second attenuation means, which attenuate, independence of said second error signal, at least one of said precedingattenuated signals in order to generate second, further attenuatedsignals, based on which said final simulcast signal is generated,wherein M is a positive integer including zero and is determinedaccording to the desired stability of the final simulcast signal and/ordegree of attenuation of each attenuation stage.
 4. Reduced peaksimulcast signal generator according to claim 3, characterized in thatsaid additional attenuation stages comprise delay elements for delayingthe preceding attenuated signals by respective time delays thatcompensate the processing time caused by said second simulcast signalgenerator and said second attenuation means.
 5. Reduced peak simulcastsignal generator according to anyone of the preceding claims,characterized in that said peak reduction stages comprise multipliersfor multiplying at least one of said broadcast signals or said precedingattenuated signals with a weighting signal.
 6. Reduced peak simulcastsignal generator according to anyone of the preceding claims,characterized in that said digital broadcast signal is a DRM signal. 7.Reduced peak simulcast signal generator according to anyone of thepreceding claims, characterized in that said analogue broadcast signalis a amplitude modulated audio broadcast signal.
 8. Reduced peaksimulcast signal generator according to anyone of the preceding claims,characterized in that said first and second simulcast signal generatorrespectively comprise a first modulator to modulate a digital signal toone sideband of a carrier of the transmission channel, a secondmodulator to modulate a correcting signal to the other sideband of thecarrier of the transmission channel, whereby said correcting signal isdetermined such that the envelope demodulation of the transmissionchannel represents the simulcast signal generator's analogue inputsignal, and a first adder for adding said both sidebands.
 9. Method forgenerating a peak-attenuated simulcast signal starting from a digitalbroadcast signal (2) and an analogue broadcast signal (1), comprisingthe following steps: generating by means of a first simulcast signalgenerator (5) from said broadcast signals (1, 2) a first error signal(6), determining if said first error signal (6) exceeds a predefinedthreshold, in case said first error signal (6) is too large, attenuatingat least one of said analogue and said digital broadcast signals (1, 2)in order to generate first attenuated signals (12, 14), generating afinal simulcast signal (28) for transmission based on said firstattenuated signals (12, 14).
 10. Method according to claim 9,characterized by delaying said broadcast signals by respective timedelays that compensate the processing time caused by said firstsimulcast signal generator and subsequent processing units.
 11. Methodaccording to claim 9 or 10, further characterized by the followingsteps: starting from said first attenuated signals, generating a seconderror signal by means of a second simulcast generator, attenuating, independence of said second error signal, at least one of said precedingattenuated signals, in order to generate second, further attenuatedsignals, based on which said final simulcast signal is generated,whereby the whole procedure is repeated M times, wherein M is a positiveinteger including zero and is determined according to the desiredstability of the final simulcast signal and/or the degree of attenuationof each attenuation stage.
 12. Method according to anyone of claims 9 to11, characterized by delaying the preceding attenuated signals byrespective time delays that compensate the processing time caused bysaid second simulcast signal generator and subsequent processing units.13. Method according to anyone of claims 9 to 12, characterized bymultiplying at least one of said broadcast signals or said precedingattenuated signals with a weighting signal.
 14. Method according toanyone of claims 9 to 13, characterized in that said digital broadcastsignal is a DRM signal.
 15. Method according to anyone of claims 9 to14, characterized in that said analogue broadcast signal is a amplitudemodulated audio broadcast signal.
 16. Method according to anyone ofclaims 9 to 15, characterized in that said simulcast signals aregenerated by combining a digital signal and an analogue signal in onetransmission channel, whereby the digital signal is modulated to onesideband of a carrier of the transmission channel, and a correctingsignal is modulated to the other sideband of the carrier of thetransmission channel, and the correcting signal is chosen such that theenvelope demodulation of the transmission channel represents theanalogue signal.
 17. Simulcast Broadcast signal combining a digitalsignal and an analogue signal in one transmission channel, characterizedin that said simulcast broadcast signal has been generated by the methodof claims 9 to
 16. 18. Simulcast broadcast signal according to claim 17,characterized in that said analogue broadcast signal is a amplitudemodulated audio broadcast signal.
 19. Simulcast broadcast signalaccording to claim 17 or 18, characterized in that said digitalbroadcast signal is a DRM signal.
 20. Computer program product,comprising computer program means carrying out the method steps asdefined in anyone of claims 9 to 16 when said computer program productis executed on a computer, digital signal processor or the like.